Maximum zs generator and rubber box

UPS · Apr 3, 2018

Can anyone point me in the right direction.

I am working for a company who do temp power for welders etc from a generator.

We have been given a load of new boards to pre check befor they go on site. Powerlock connectors in and then D63 on a commando sockets.

I have put the maximum zs of the sockets from the table I usually use. But have just recieved an email from a member of management saying that we need to discuss the max zs values I have used (0.14).

Am I missing something?

Pete999 · Apr 3, 2018

UPS said:
Can anyone point me in the right direction.

I am working for a company who do temp power for welders etc from a generator.

We have been given a load of new boards to pre check befor they go on site. Powerlock connectors in and then D63 on a commando sockets.

I have put the maximum zs of the sockets from the table I usually use. But have just recieved an email from a member of management saying that we need to discuss the max zs values I have used (0.14).

Am I missing something?

Well yes you are, you have only noted the Maz Zs for the particular OCPD you are using, not the actual Zs of the instalation, Zs = Ze+ (R1+R2) So a Zs measurement needs to be taken at the point of utilisation, ergo the socket you are supplying.

Wilko · Apr 3, 2018

Not an expert here but to add to @Pete999 thoughts - would the gen set be able to provide the 1260A required to trip a D63 instantly or even 630A for 5 sec? So, perhaps it's earth fault protection is via RCD?

Pete999 · Apr 3, 2018

Wilko said:
Not an expert here but to add to @Pete999 thoughts - would the gen set be able to provide the 1260A required to trip a D63 instantly or even 630A for 5 sec? So, perhaps it's earth fault protection is via RCD?

Yeah could be Wilko

Pete999 · Apr 3, 2018

UPS said:
Can anyone point me in the right direction.

I am working for a company who do temp power for welders etc from a generator.

We have been given a load of new boards to pre check befor they go on site. Powerlock connectors in and then D63 on a commando sockets.

I have put the maximum zs of the sockets from the table I usually use. But have just recieved an email from a member of management saying that we need to discuss the max zs values I have used (0.14).

Am I missing something?

Is the circuit orotected by the D63 a distribution circuit, if so it could be argued that the Zs talked about, coul be the ZS@the DB (Ze of the distribution circuit) or am I overthinking the issue, have been acused of that before.

davesparks · Apr 3, 2018

Are these standard rental generators?
Normally you’d be using the variable earth leakage protection on the generator set and any other earth leakage protection in your rubber boxes for your earth fault protection. The ocpds will only be providing over current protection.

Generators rarely have the ability to deliver the fault currents needed to use the mcb for fault protection. Quite often the generator will stall or shutdown on engine under-speed before any of the protective devices operate.

UPS · Apr 3, 2018

The generator is a 800kva and the earth leakage is left off.

The gen can produce the required current to trip the breaker on instantaneous.

The circuit is not a disto circuit it is being used to feed welding equipment.

davesparks · Apr 3, 2018

Pete999 said:
Well yes you are, you have only noted the Maz Zs for the particular OCPD you are using, not the actual Zs of the instalation, Zs = Ze+ (R1+R2) So a Zs measurement needs to be taken at the point of utilisation, ergo the socket you are supplying.

Generators don’t strictly have a Ze, I usually see it expressed as Zgen if anything.

davesparks · Apr 3, 2018

UPS said:
The generator is a 800kva

800kVA prime or standby?
What is the maximum fault current it can deliver and in what time?
How recently was the engine serviced and is it in full working order?

There’s more to consider with a generator supply than there is with a DNO supply.

UPS · Apr 3, 2018

The generator when tested produced a fault current of 2.8Ka

The generator is running prime.

The generator has been serviced every 450hours

I have recently started working for this company after 14 years in fixed wiring, but maybe it doesn’t transfer over like fixed wiring?

davesparks · Apr 3, 2018

UPS said:
The generator when tested produced a fault current of 2.8Ka

The generator is running prime.

The generator has been serviced every 450hours

I have recently started working for this company after 14 years in fixed wiring, but maybe it doesn’t transfer over like fixed wiring?

Is that the actual measured current under fault conditions? Or is that the fault current given by a test meter after a Zs test?

It’s not completely different to fixed wiring, but there are extra considerations with generators.

Generators react to the load applied a lot more than a supply from the DNO does. The DNO supply has several massive alternators in power stations across the country keeping it stable, the generator has just its own engine and alternator.
The engine will slow down when a large load is applied, so they can rarely deliver large instantaneous currents, it takes time to speed up the engine.

You can kind of relate a short circuit to trying to pull away in 5th gear , it don’t like it and will probably stall.

UPS · Apr 3, 2018

That is the value given after doing a test with a meter.

The generator is fitted with a PMG I’m sure this was meant to stop this happening or at least help?

So let’s cut to the chase am I missing something or do the values on the chart still count on a generator fed installation?

Thank you for every ones input.

Risteard · Apr 3, 2018

As above the generator is unlikely to be able to deliver sufficient current into a fault for disconnection either at all or at least within the required time.

UPS · Apr 3, 2018

[ElectriciansForums.net] Maximum zs generator and rubber box

Risteard said:
As above the generator is unlikely to be able to deliver sufficient current into a fault for disconnection either at all or at least within the required time.

This is the value I have from the generator this doesn’t seem dissimilar from what I would expect from a fixed install.

westward10 · Apr 3, 2018

PM Generators have been proven to sustain short circuit current during fault conditions which prevents the field collapsing.

Maximum zs generator and rubber box

UPS

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Pete999

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Wilko

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Pete999

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Pete999

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davesparks

UPS

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davesparks

davesparks

UPS

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davesparks

UPS

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Risteard

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UPS

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westward10

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